Sounds like pre-ignition to me. If it is you would have the noise when you step on the gas or put a load on the engine, but not at idle. Is that what's happening? What octane gas are you using? A octane rating of 89 is required at the least. On the passenger side of the engine below the plugs is a knock sensor. It looks like a engine temp sensor. Check the wiring and the sensor. The knock sensor will retard the timing if it hears a knock and is working right. If low octane gas is used or the timing is set to high and the knock sensor is bad, you'll have that sound. Check your timing and the sensor first, its an easy fix if thats the problem. The octane of the gas used, can be boosted with an additive untill you run the tank down and fill it with 89 octane. If you hear that sound at idle, it could be lifters. That would be more of a ticking sound tho. As far as the cats being bad and rattling with the low mileage is beyond my thinking. I've never came across that happening in my experiences, but thats not to say its unlikely. Does your check engine light come on? Have you checked to see if any codes have been set? If your timing, gas and the sensor and wiring check out Ok, then look else where. I think what I suggested is the place to start and eliminate that first. Others may have different opinions than mine, so take it all into consideration. One thing I was wondering about at the last minute here, was to ask if you had engine run on after turning the key off. That would if it does that, lead me to think I'm right. Noise is sometimes hard to track down, because it can travel, good luck and I hope this helps you. Be sure to let us know what you find and what it was. Bill
